In recent years, hybrid vehicles, which have two types of drive sources, an internal combustion engine and an electric motor, have been put in practical use to improve fuel efficiency and emission. As a hybrid system functioning as a drive source for such a hybrid vehicle, a system has been put in use that includes an internal combustion engine, two electric motors, and a power distribution mechanism formed by a planetary gear mechanism. Such a hybrid system controls the rotational speed of the internal combustion engine via power running operation and regeneration operation of the first electric motor. The system also assists the drive torque and the braking torque of the vehicle via power running operation and regeneration operation of the second electric motor.
A type of hybrid vehicles has been known in which a hybrid system is connected to a geared transmission that shift gears via engagement and disengagement of frictional engagement elements such as clutches and brakes. In such a hybrid vehicle, when shifting gears of the geared transmission, synchronization of the frictional engagement elements of the transmission can be achieved by controlling the input rotational speed of the transmission using an electric motor.
Conventionally, a device disclosed in Patent Document 1 has been known as a shift control device for a hybrid vehicle. In the shift control device disclosed in Patent Document 1, the change rate of a target rotational speed of a second electric motor is variably set in accordance with the degree of progress of shifting at power-on downshift of a geared transmission. This prevents the torque from being undesirably lowered during shifting.
Also, a device disclosed in Patent Document 2 has also been known in the art as a shift control device for a hybrid vehicle. In the shift control device disclosed in Patent Document 2, it is determined, in accordance with the traveling state of a vehicle, which one of response to shifting and reduction of synchronization shock at downshift should be given priority. According to the result of the determination, the change gradient (change rate) of the engine speed during downshift, that is, the change gradient of the input rotational speed of the geared transmission is varied. When the deceleration of the vehicle is great, it is determined that the response to shifting is given priority, and the change gradient of the input rotational speed of the geared transmission during downshift is increased.